runtime: add Gosched to TestSelectStackAdjust loop

Give the runtime more of a chance to do other work in a tight loop.

Fixes #34693

Change-Id: I8df6173d2c93ecaccecf4520a6913b495787df78
Reviewed-on: https://go-review.googlesource.com/c/go/+/210217
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/chan_test.go

@@ -719,6 +719,7 @@ func TestSelectStackAdjust(t *testing.T) {
 		if after.NumGC-before.NumGC >= 2 {
 			goto done
 		}
+		runtime.Gosched()
 	}
 	t.Fatal("failed to trigger concurrent GC")
 done: